"It is my pleasure to announce my support for America's Mayor, Rudy Giuliani, a proven leader who is not afraid of what lies ahead and who will cast a hopeful vision for all Americans," Robertson said during a news conference with Giuliani in Washington.

Giuliani, former mayor of New York, supports abortion rights and homosexual rights despite the Republican Party's official stands against such issues.

Robertson, who made an unsuccessful presidential bid in 1988, is also the founder of the Christian Broadcasting Network, the Christian Coalition and Regent University. Shortly after the terrorist attacks of September 11, 2001, Robertson claimed in a statement that the attacks occurred because Americans had insulted God and lost the protection of heaven by “allowing abortion and rampant internet pornography."

Robertson did not mention his differences with Giuliani on social issues in Wednesday's statement.

Senator John McCain, another Republican presidential candidate, was asked about Robertson's endorsement at a news conference with Senator Sam Brownback in Dubuque, Iowa. He said: "Every once in a while, I'm left speechless. This is one of those times."

Some social conservative organizations were also taken aback by Robertson's endorsement.

"We are stunned and extremely disappointed," said the Republican National Committee for Life in a Wednesday notice. The committee suggested voters contact Robertson to ask him the reason for his decision.